1. Field of the Invention
The present invention relates to a photocathode (photoelectron emitting surface) for emitting photoelectrons in response to photon incidence.
2. Related Background Art
A photocathode comprising a light absorbing layer and an electron emitting layer provided on a semiconductor, and means for applying an electric field between these light absorbing layer and electron emitting layer is disclosed, for example, in Japanese Patent No. 2923462 (Reference 1). This photocathode comprises a substrate composed of InP. A light absorbing layer composed of InGaAs having a thickness of 2 μm is formed on the upper layer of the substrate, while a p-type InP electron emitting layer having a thickness of 0.7 μm is formed on the light absorbing layer. Further, a mesh-shaped electrode comprising an n-type InP layer and a Ti metal layer for providing a potential to this n-type InP layer is formed on the p-type InP electron emitting layer.
A p-n junction is formed between the n-type InP layer and the p-type InP electron emitting layer and between the latter layer and the light absorbing layer. An electric field is applied between the light absorbing layer and the electron emitting layer by an electric power supply, a wiring, and an electrode composed of AuZn. In this photocathode, the mesh electrode has a width of 2 μm and an electrode spacing of 4 μm. Cesium oxide is applied to the exposed part of the surface of the p-type InP electron emitting layer, so as to reduce the work function of the surface of the p-type InP electron emitting layer. The photocathode is sealed in vacuum, and accommodated in a vessel having a light incident window. Further, electrons emitted from the photocathode reach a collector electrode provided.